1. Field of the Invention
The present invention relates in general to telephone systems utilizing Tl type carrier communication links, and more particularly to an arrangement for testing the digital trunk units terminating these links to ensure their proper operation.
2. Description of the Prior Art
Digital transmission such as used on a Tl type span line is effected by changing temperatures. As the temperature on a Tl span line increases the delay associated with the line increases. When the temperature decreases the delay decreases. With a constant temperature, PCM data is sent over a Tl span to a Digital Trunk Unit (DTU) at the rate of 1.544 Mhz. When the temperature is increasing the PCM data appears to be sent to the DTU at a slightly lower rate than usual. As the temperature decreases the PCM data appears to be sent at a rate slightly higher than 1.544 Mhz. The DTU contains a line compensator which consists of a first in-first out buffer to allow for discrepancies in the send and receive rates. If PCM data is sent to the DTU at a slightly slower rate than the DTU receive rate (1.544 Mhz) this buffer will eventually empty and PCM data will be lost, or at least mutilated in that the equipment will "stuff pulses" to make up the void. On the other hand if PCM data is sent to the DTU slightly faster than 1.544 Mhz the buffer will eventually fill up and PCM data will be lost. The change in delay for a 400 mile Tl span line with a 70.degree. F. change in temperature is the equivalent of 96 bits of PCM data. This change is easily handled by the line compensator.
To determine that the DTU is operating properly, equipment capable of simulating the effects of temperature upon the transmission media is required.
In the past, an analog circuit was used to simulate temperature effects. The analog circuit contained a phase shifter which shifted a clock signal 360.degree. to the right for an increase in time and then switched to an unshifted clock while the phase shifter shifted 360.degree. to the left. Every time this cycle occurred a clock pulse would be lost. After 96 of these pulses were lost the Digital Trunk Unit would fail. This process required 77 minutes for each trunk to fail. In the normal testing mode the circuit was allowed to run until the Digital Trunk Unit failed.
Other problems with analog circuits have to do with adjustments to the analog phase shifter. A ramp voltage is presented to the phase shifter to determine the amount of phase shift. When the ramp reaches a point corresponding to a 360.degree. phase shift the ramp is reset until it reaches a point corresponding to a 0.degree. phase shift. The adjustments to set the 360.degree. and 0.degree. points on the ramp are very critical and because of this are difficult to adjust. Another of the problems have to do with noise on the ramp voltage waveform. The adjustments are so critical that they cannot be made when the card is on a slot extender.